Modulator circuit for high power linear beam tube

ABSTRACT

A high power linear beam tube; to be modulated, such as a highpower klystron, has its cathode directly connected with the depressed collector electrode of a modulator tetrode tube and its anode connected to the accelerating electrode of the tetrode modulator tube. The modulator tetrode tube includes a gridded convergent flow linear beam electron gun and a mirror image, substantially 100 percent depressed collector. The control grid is non-intercepting grid with and provides a voltage gain of 20 to 50 and a relatively high-power gain, as of 20 to 40 db. The electron gun includes a massive non-intercepting accelerating anode disposed between the control grid and the depressed collector. When the beam traverses the accelerating anode to the depressed collector, the collector is lowered toward cathode potential, causing an equivalent current to flow in the thermionic diode load. The beam perveance of the load tube is preferably substantially equal to the perveance of the electron gun of the tetrode modulator tube.

United States Patent [191 Giebeler Apr. 2, 1974 MODULATOR CIRCUIT FORHIGH POWER [57] ABSTRACT LINEAR BEAM TUBE A high power linear beam tube;to be modulated, such [75] Inventor: Robert Henry Giebeler, Sunnyvale,as a high-power klystron, has its cathode directly con- Calif. nectedwith the depressed collector electrode of a [73] Assign: varianAssociates, Palo Alto, Calif modulator tetrode tube and its anodeconnected to the accelerating electrode of the tetrode modulator tube.[22] Filed: Aug. 24, 1972 The modulator tetrode tube includes a griddedconvergentflow linear beam electron gun and a mirror [21] Appl' 283433image, substantially 100 percent depressed collector.

The control grid is non-intercepting grid with and prol5 l Cl 315/529,vides a voltage gain of 20 to 50 and a relatively high- 332/58, 315/35power gain, as of 20 to 40 db. The electron gun in- [51] Int. Cl H01j25/12, H01 j 25/16 cludes a massive non-intercepting accelerating anode[58] Field of Search 332/7, 13, 58; 3l5/3.5 X, disposed between thecontrol grid and the depressed 315/529, 5.38, 3.5 collector. When thebeam traverses the accelerating anode to the depressed collector, thecollector is low- [56] References Cited ered toward cathode potential,causing an equivalent UNITED STATES PATENTS current to flow in thethermionic diode load. The 2,719,954 10 1955 Palluel et a1. 332 7 beamperveance of the load tube is preferably subsum' 2,842,742 7/1958 heist332/7 tially equal to the perveance of the electron gun of the 3,225,31412/1965 Rambo 332/7 telrode modulator tube- 2,338,237 l/l944 Fremlin332/7 3,453,482 7/1969 Preist 315/529 Primary ExaminerRudolph V. RolinecAssistant Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or Firm-Stanley Z. Cole; D. R. Pressman; Robert K.Stoddard 5 Claims, 5 Drawing Figures LINEAR BEAM TUBE LOAD d PULSER sCONTIlOL I.5KV GRI D ,25

BIAS

v BEAM POWER e IZO-IGOKV SUPPLY b ATENTEDAPR 2 I974 FIG. I PRIOR ART QI20- |60KV saw '1 ur LINEAR BEAM TUBE LOAD W PULSER KV GRID BI AS BEAMPOWER CONTROL SUPPLY E MODULATOR CIRCUIT FOR HIGH POWER LINEAR BEAM TUBEGOVERNMENT CONTRACT The invention herein described was made in thecourse of or under a contract or sub-contract with the U.S. Departmentof the Air Force.

DESCRIPTION OF THE PRIOR ART Heretofore, high-power in excess ofthermionic diode loads, such as klystron amplifiers, traveling wavetubes and the like, have been pulse-modulated by means of a hard tubetriode modulator series-connected such that the plate of the triode wasconnected to the thermionic cathode of the load and the cathode of thetri ode modulator was connected to the negative terminal of the beampower supply. The positive terminal of the beam power supply wasconnected to the anode of the load. The load was modulated by applying apulse to the control grid of the triode modulator tube for turning onthe modulator tube and causing the cathode of the load to be depressedto the potential of the negative terminal of the beam power supplythrough the conducting modulator tube.

The problem with a hard tube triode modulator, in the aforecitedcircuit, is that an arc in the triode modulator tends to pull the triodeinto a more conductive state, thereby increasing the current through theload. In addition, arcs within the triode tube tend to migrate to thegrid because the grid is in a position of maximum gradient (normalcoaxial triode configuration,) thereby tending to destroy the grid ofthe modulator tube.

Moreover, an arc in the load tube causes the triode modulator tube to besubjected to full beam voltage. Because the triode grid is positioned ina region of maximum gradient, an arc in the load tube tended to increasethe probability of producing a corresponding arc in the modulator tube.

Another problem with the hard tube triode modulator is that it isrelatively inefficient. The voltage drop across the modulator tubeduring its conducting intervals is approximately percent of the voltagebeing switched. This is because'the grid-to-place spacing has to belarge to hold off the high grid-to-plate voltage. A large grid-to-platespacing results in a relatively large voltage drop when the tube isconducting.

It is also known from the prior art that a high-power tetrode beam tubeemploying a depressed flytrap beam collector could be made to have highefficiency by shaping the equipotential surfaces at the mouth of thecollector, when operating at depressed potential, to be approximately amirror image of the equipotential surfaces at the thermionic cathodeemitter in order to attain laminar electron flow of uniform currentdensity as the beam expands into the collector. Such a high-power beamtube is disclosed and claimed in U.S. Pat. No. 3,453,482, issued July 1,1969, and assigned to the same assignee as the present invention.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved modulator circuit forhighpower thermionic linear beam tube loads.

In one feature of the present invention, an improved modulator circuitis provided wherein a high-power tetrode of the type described in theabovecited U.S. Pat.

No. 3,453,482 is connected in series with a beam power supply and theload to be modulated, such that the load is connected between thedepressed collector and accelerating anode of the high-power tetrode,whereby a more reliable and efficient modulator circuit is obtained.

In another feature of the present invention, the drive potentialsupplied to the control grid of the tetrode modulator tube is adjustedsuch that the preveance of the electron gun of the modulator tube isadjusted to be approximately equal to the perveance of the electron gunof the load, whereby efficient modulator tube operation is obtained.

In another feature of the present invention, the thermionic load is ahigh-power linear beam tube having a beam microperveance within therange of 0.75 to 3.5 and the modulator tube has an electron gunmicroperveance falling within the same range.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram,partly in block diagram form, of a prior art thermionic load modulatorcircuit,

FIG. 2 is a schematic circuit diagram, partly in block diagram, form, ofa thermionic load modulator circuit incorporating features of thepresent invention,

FIG. 3 is a plot of load current in amps vs. grid drive in RV for twovalues of beam power supply voltage E,, and depicting the grid drivecharacteristics for the modulator circuit of FIG. 2,

FIG. 4 is a plot of load current in amperes and efficiency in percentvs. modulator tube cathode voltage E, in RV depicting the efficiency ofthe modulator tube and the load at a nominal kV load voltage E,, for themodulator circuit of FIG. 2, and

FIG. 5 is a plot similar to that of FIG. 4 depicting modulator tubeefficiency and load current at a nominal I38 kv load voltage E for themodulator circuit of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a prior art hard tube triode modulator circuit for modulating athermionic linear beam tube load 1, such as a highpower klystronamplifier. The load 1 includes a beam collector 2 and a non-interceptingaccelerating electrode or anode 3 operating at ground potential which isalso the positive terminal of a beam power supply 4 supplying a beamvoltage E,,, as of l2() to l60 KV.

A thermionic cathode emitter 5 of the load 1 is connected to thenegative terminal of the beam power supply 4 via the plate-to-cathodecircuit of a hard tube triode modulator tube 6. A grid pulser 7 pulsesthe grid 8 of the triode modulator tube 6 positive with respect to itsthermionic cathode 9 for turning on the modulator tube 6 and causing theplate II and the modulator 6 to be dropped to substantially the negativepotential of the beam power supply, thereby depressing the potential onthe cathode 5 of load 1 to turn on the beam in load 1.

The modulator tube 6 is typically of the coaxial geometricalconfiguration wherein the thermionic cathode emitter 9 comprises aseries of longitudinally directed filaments centrally disposed of thetube 6 and which includes the outer cylindrical plate 11 tube 6. Acylindrical cage-shaped grid structure 8 is closely spaced to thethermionic emitter 9 for controlling the current to the cylindricalplate 11.

The problem with this sort of modulator tube is that the electron beamis collected on the plate 11 in a region of relatively high electricfield such that ions by bombardment of the plate can backstream to thecontrol grid 8, which is also disposed in a region of relatively highgradient, to initiate an are between the control grid 8 and the plate11. An arc tends to turn the tube on harder such that more current isdrawn by the modulator tube and excessive current may be drawn by load1.

In addition, because the grid-to-plate spacing has to be high to holdoff the high plate-to-grid voltage, the voltage drop through the tube,when conducting, is relatively high, resulting in a relatively lowefficiency for the modulator tube. A typical efficiency is approximately80 percent, there being a 20 percent voltage drop between the grid andplate of the modulator tube.

Moreover, the grid drive power, due to the relatively large beaminterception on the grid, is relatively high. For example, to pulsemodulate megawatts peak output power, the peak grid drive power, due togrid interception, is approximately 40 kW. Thus, the grid pulser 7 mustbe capable of supplying a relatively high peak power to the grid circuitof the modulator tube.

Another disadvantage of the hard tube modulator circuit of FIG. 1 isthat when an arc occurs in the load tube, excessive voltage appearsacross the grid-to-plate circuit of the modulator tube which mayinitiate an arc in the modulator tube. Thus the modulator tube is notprotected from shorts in the load tube.

Referring now to FIG. 2, there is shown a modulator circuit formodulating a high-oower thermionic linear beam tube load 1 andincorporating features of the present invention. More particularly, thecircuit of FIG. 2 is similar to that of FIG. 1 except that the triodehard tube modulator 6 has been replaced, in the circuit of FIG. 2, witha high efficiency tetrode switch tube of the type disclosed and claimedin the aforecited US. Pat. No. 3,453,482. The disclosure of US. Pat. No.3,453,482 is hereby expressly incorporated by reference for a detaileddescription of the tetrode modulator tube 12.

Briefly, the modulator tube 12 includes a thermionic cathode emitter 13,preferably of the oexide coated type, having a spherically concavecathode emitting surface 14, preferably dimpled to form a multiplicityof individual concave lesser cathode emitters in the manner as disclosedand claimed in US. Pat. No. 3,558,967, issued Jan. 26, 1971, andassigned to the same assignee as the present invention.

A shadow grid is disposed overlaying the emitting surface 14 of thethermionic cathode emitter 13. The apertures in the shadow grid 14 arealigned with the spherically concave lesser cathode emitting surfaces.

The shadow grid 15 is operated at essentially the same potential as thethermionic cathoce emitter 13 for suppression of emission from thecathode in the undimpled region of the cathode shadowed by the grid 15.

A spherically concave control grid 16 is disposed overlaying the shadowgrid 15 with the apertures of the control grid being aligned inregistration with the apertures in the shadow grid 15. The control grid16 is spaced from the cathode emitting surface 14 by relatively shortdistance, as of 0.039 inch, to define a multi plicity of Pierce gunswhen a positive potential, relative to the cathode, is applied to thecontrol grid 16. The control grid 16 is supported in electricallyinsulative relation relative to the thermionic cathode, as by aninsulator mounted to a surrounding focus electrode 17 operating atcathode potential.

A relatively massive centrally apertured accelerating electrode 18, asof copper, forms the anode of the composite electron gun comprisinganode 18 and cathode 13. The composite electron gun is preferably of thePierce design for laminar convergent electron flow from the thermioniccathode emitter surface 14 through a central aperture 19 in theaccelerating electrode 18 to a depressed beam collector structure 21 atthe terminal end of the beam path. The diameter of the acceleratingaperture 19 in the accelerating electrode 18 is preferably substantiallysmaller than the crosssectional dimension of the beam as it leaves theemitting surface 14 such that there is a laminar convergent electronflow through the aperture 19 in the accelerating electrode 18. Also thelength of the passageway 19 through the accelerating electrode 18 ispreferably longer than its transverse dimensions such that the cathodeemitter 13 is shielded from the electric fields of the depressedcollector 21.

The collector 21 is of the flytrap type wherein the collector cavityincludes a constricted mouth portion 22 forming the entrance passagewayinto the collector cavity 21. A focus electrode structure 23 is providedat the lip of the beam entrance aperture 22 which cooperates with themutually opposed surfaces of the accelerating electrode 18 such that aseries of dish-shaped equipotential surfaces are formed in the regionbetween the accelerating anode 18 and the mouth 22 of the beam collector21 which is substantially a mirror image of the similar equipotentialsformed between the concave emitting surface 14 of the thermionic cathodeemitter l3 and the mutually opposed surfaces of the accelerating anode18.

In addition, a tapered conductive probe 20 is preferably providedcoaxially within the collector cavity 21 and terminating at a pointsubstantially in the plane of the mouth 22 to further aid in obtaininglaminar electron flow of uniform current density across the beam as itenters the mouth 22 of the depressed collector 21. The probe isdisclosed and claimed in my copending US. application Ser. No. 283,431,filed Aug. 24, l972 and; assigned to the same assignee of the presentinvention.

Thus, in this manner, substantially laminar flow for the electron streamis obtained from the cathode emitter 13 through the accelerating anode18 and into the beam collector 21 to minimize reflection of electronsback to the accelerating electrode 18 when the potential of the beamcollector 21 has been depressed to nominally the potential of thethermionic cathode emitter 13, as is obtained when the modulator tube 12has been turned on and is conducting maximum current through the load 1.The modulator tube 12 includes an evacuated envelope 24.

The modulator tube 12 is normally biased to an off (non-conducting)state via a d.c. negative potential, as of 1.5 kV, supplied to thecontrol grid from a control grid bias supply connected between thenegative terminal of the beam power supply 4 and the control grid 16.The grid pulser 7 is series connected with the control grid bias supply25 and the control grid 16 for supplying a positive grid pulse E as of lto 5 kV, to the control grid 16. The positive grid pulse of voltage E issuperimposed upon the d.c. negative control grid bias, as of 1.5 kV,such that the effective positive control grid voltage E is thedifference between the d.c. negative control grid bias and the positivegrid pulse E In operation the pulse input grid voltate E is adjustedsuch that electron gun perveance of the modulator tube 12 isapproximately equal to or slightly less than the beam perveance of thegun of load 1. When this is achieved, maximum efficiency for themodulator tube 12 is obtained because the magnitude of the voltage onthe beam collector is slightly less than the magnitude of the beamsupply voltage.

An advantage of this tetrode modulator circuit when used with athermionic linear beam tube load is that the operating characteristics,such as efficiency, are constant over a wide range of power levels,provided appropriate adjustment of beam voltage and grid drive is made.

The constant efficiency is the result of the fact that the loadimpedance presented by a given perveance load is the optimum impedancefor the tetrode modulator tube due to electron optics considerationswhich are optimum at a geven beam perveance. The degree of collectordepression of the modulator tube 12 which is allowable withoutreflecting electrons to the accelerating electrode determines themodulator tube voltage drop and, therefore, the efficiency of themodulator The p. of the grid 16 is generally in the range of 20 200 witha power gain of 20 to 40 db.

Referring now to FIGS. 3-5 there is shown the typical operating curvesfor a typical modulator circuit of FIG. 2. At a load voltage E ofapproximately 120 kV FIG. 4 and a load current of 82 amps, theefficiency of the modulator circuit varies from 93 percent to 84 percentas a function of the power supply voltage E FIG. 5 shows a plot ofdynamic impedance and efficiency for an operating condition of 100 ampsload current at 138 kV load voltage E, with a beam voltage 13,, cm themodulator tube 12 of between 145 and 160 kV.

One advantage of the series modulator tube is a relatively high dynamicimpedance which results in a video voltage E which is relativelyindependent of fluctuations in supply voltage E,,.

Another advantage of the modulator circuit of FIG. 2 is that the loadtube 1 is protected from damage in the event of an internal arc ineither the load tube 1 or the modulator tube 12 since the load tube 1 isisolated from the d.c. power supply 4 by the massive accelerating anode18 of the modulator tube 12, which is operated at ground potential. Themaximum arc current in the load tube 1 is limited to the current drawnthrough the modulator tube 12.

The rise time of the load voltage E follows closely the rise time of thegrid drive voltage E since the only capacity to be charged is thecapacity to ground of the modulator tube collector 21 pf), the electrongun of the thermionic load 1 (30 pf), and the filament transformer (notshown) which can be as low as 30 pf. This total capacity of pf can becharged by the beam current in nanoseconds.

The recommended beam voltage E is 1 10 percent of the video output levelE since this allows i 5 percent variation of beam voltage E for rippleregulation and drop with a reasonable efficiency and a minimumdissipation.

Another advantage of the modulator circuit of FIG. 2 is that theefficiency is relatively independent of the power level at which theload 1 is supplied. For example, if it is desired to operate the load 1at a lower power level, the beam voltage E, of the power supply 4 islowered and the grid drive voltage levels are readjusted'such that theelectron gun perveance ofmodulator tube 12 is approximately equal tothat of load 1 being modulated.

Another advantage of the tetrode modulator circuit of the presentinvention is that the massive accelerating electrode 18 operating atground potential serves as an excellent r.f. shield for shielding thecontrol grid 16 from the collector 21, thereby eliminating thedegradation in gain called Miller effect experienced with prior arttriode modulators. In addition, the tetrode may be installed in such amanner to shield the grid pulse modulator 7 from the output videovoltage.

Although the description, thus far, has described the load 1 as amicrowave linear beam tube, such as a klystron or a traveling wave tube,it is also useful for modulating the power output of other types ofthermionic loads which have diode-like electrical characteristics,

such as magnetrons, electron guns for exciting plasma discharges inhigh-power laser tubes, and other types of electron guns. There does notappear to be a theoretical limit to the peak or average power outputthat may be modulated by the modulator circuit of FIG. 2, since themodulator tube 12 may be merely physically scaled to larger or smallerdimensions to accommodate a wide range of power levels.

While the above description contains many specificities, these shouldnot be construed as limitations upon the scope of the invention, butmerely as an exemplification of the preferred embodiments thereof. Theintended scope of the invention is indicated by the following claims andtheir legal equivalents.

What is claimed is:

1. A high power modulator circuit, comprising:

high power beam tube means having a thermionic cathode and an anodespaced therefrom to define an operable electron gun for generating anelectric beam which is to be modulated;

modulator tube means having a thermionic cathode,

a centrally apertured, substantially nonintercepting accelerating anodespaced from said cathode for drawing electrons through said acceleratingaperture for forming a high power electron beam, collector means forcollecting said electron beam, said collector means operating at adepressed potential with respect to said anode, and a substantiallynon-intercepting control grid interposed between said anode and saidcathode for controlling said electron beam;

power supply means for supplying direct current power to said beam tubemeans and said modulator tube means;

said modulator tube means being series-connected with the beam circuitsof said beam tube means and said power supply means, the positiveterminal of said power supply means being connected to said anode ofsaid modulator tube means, and said thermionic cathode of said beam tubemeans being directly connected electrically to said depressed potentialcollector means of said modulator tube means, whereby said beam tubemeans forms the load of said modulator tube means; and means for pulsingsaid control grid of said modulator tube means positive with respect tosaid cathode thereof for modulating the beam current of said modulatortube and hence the beam current of said beam tube means. 2. Theapparatus of claim 1 including control grid bias means connected forsupplying a direct current bias potential to said control grid of saidmodulator tube, said bias potential being sufficiently negative relativeto the potential of said thermionic cathode of said modulator tube suchthat in the presence of a turn-on pulse of predetermined positivepotential applied to said control grid relative to said thermioniccathode of said modulator tube means, the perveance of the electron gunof said modulator tube means will be substantially equal to theperveance of the electron gun of said beam tube means.

3. The apparatus of claim 1 wherein said thermionic cathode of saidmodulator tube means has a concave emitting surface facing said centralaperture in said accelerating anode of said modulator tube means, saidcentral aperture of said accelerating anode being unobstructed and ofsubstantially less cross-sectional area '8 than said concave emittingsurface of said cathode emitter, and wherein said collector meansincludes a cavity having a constricted beam entrance mouth por tionfacing said central aperture of said accelerating anode for passage ofthe beam into said collector means while restricting the backstreamingof secondary electrons from said collector cavity, back toward saidaccelerating anode, and wherein the portions of said collector meansfacing said accelerating anode are shaped relative to the shape of theopposed surfaces of said accelerating anode to provide a series ofgenerally concave equipotentials in the space between saidaccelcrating-anode and said collector means, which series ofequipotentials are generally a mirror image of a similar series ofequipotentials in the space between said cathode and said acceleratinganode when said tube is conducting maximum rated beam current into saidbeam tube means.

4. The apparatus of claim 2 wherein said beam tube means comprises ahigh frequency linear beam tube including said thermionic cathode, anon-intercepting centrally apertured anode spaced from said thermioniccathode for forming and projecting a beam of electrons over an elongatedbeam path, a beam collector structure at the terminal end of said beampath for collecting said beam, and a high frequency electrical circuitdisposed along the beam path intermediate said anode and said collectorin electromagnetic wave energy exchanging relation with said beam.

5. The apparatus of claim 2 wherein the electron gun of said beam tubemeans has a microperveance falling within the range of 0.75 to 3.5

1. A high power modulator circuit, comprising: high power beam tubemeans having a thermionic cathode and an anode spaced therefrom todefine an operable electron gun for generating an electric beam which isto be modulated; modulator tube means having a thermionic cathode, acentrally apertured, substantially non-intercepting accelerating anodespaced from said cathode for drawing electrons through said acceleratingaperture for forming a high power electron beam, collector means forcollecting said electron beam, said collector means operating at adepressed potential with respect to said anode, and a substantiallynon-intercepting control grid interposed between said anode and saidcathode for controlling said electron beam; power supply means forsupplying direct current power to said beam tube means and saidmodulator tube means; said modulator tube means being series-connectedwith the beam circuits of said beam tube means and said power supplymeans, the positive terminal of said power supply means being connectedto said anode of said modulator tube means, and said thermionic cathodeof said beam tube means being directly connected electrically to saiddepressed-potential collector means of said modulator tube means,whereby said beam tube means forms the load of said modulator tubemeans; and means for pulsing said control grid of said modulator tubemeans positive with respect to said cathode thereof for modulating thebeam current of said modulator tube and hence the beam current of saidbeam tube means.
 2. The apparatus of claim 1 including control grid biasmeans connected for supplying a direct current bias potential to saidcontrol grid of said modulator tube, said bias potential beingsufficiently negative relative to the potential of said thermioniccathode of said modulator tube such that in the presence of a turn-onpulse of predetermined positive potential applied to said control gridrelative to said thermionic cathode of said modulator tube means, theperveance of the electron gun of said modulator tube means will besubstantially equal to the perveance of the electron gun of said beamtube means.
 3. The apparatus of claim 1 wherein said thermionic cathodeof said modulator tube means has a concave emitting surface facing saidcentral aperture in said accelerating anode of said modulator tubemeans, said central aperture of said accelerating anode beingunobstructed and of substantially less cross-sectional area than saidconcave emitting surface of said cathode emitter, and wherein saidcollector means includes a cavity having a constricted beam entrancemouth portion facing said central aperture of said accelerating anodefor passage of the beam into said collector means while restricting thebackstreaming of secondary electrons from said collector cavity backtoward said accelerating anode, and wherein the portions of saidcollector means facing said accelerating anode are shaped relative tothe shape of the opposed surfaces of said accelerating aNode to providea series of generally concave equipotentials in the space between saidaccelerating anode and said collector means, which series ofequipotentials are generally a mirror image of a similar series ofequipotentials in the space between said cathode and said acceleratinganode when said tube is conducting maximum rated beam current into saidbeam tube means.
 4. The apparatus of claim 2 wherein said beam tubemeans comprises a high frequency linear beam tube including saidthermionic cathode, a non-intercepting centrally apertured anode spacedfrom said thermionic cathode for forming and projecting a beam ofelectrons over an elongated beam path, a beam collector structure at theterminal end of said beam path for collecting said beam, and a highfrequency electrical circuit disposed along the beam path intermediatesaid anode and said collector in electromagnetic wave energy exchangingrelation with said beam.
 5. The apparatus of claim 2 wherein theelectron gun of said beam tube means has a microperveance falling withinthe range of 0.75 to 3.5